Such horn antennas are found, for example, in microwave measuring devices of process and automation, measurements technology. These measuring devices are frequently used in automation and process control technology, in order to ascertain the process variable, fill level, in a container. Endress+Hauser, for instance, produces and distributes measuring devices under the mark Micro Pilot. These devices work according to the travel-time measuring method and serve for determining and/or monitoring a fill level of a medium in a container. In the travel-time measuring method, for example, high-frequency pulses, or radar wave pulses, are emitted via an antenna, and echo waves reflected on the surface of the medium are received back, following a distance-dependent travel time of the signal. From the time difference between emission of the high-frequency pulses and receipt of the reflected echo signal, the fill level can be ascertained. The so-called FMCW (Frequency Modulated Continuous Waves) method is likewise performable in this connection with the aforementioned principle of fill level measurement and the aforementioned apparatus.
Horn antennas filled with microwave-transmissive, dielectric material for improving the durability of the horn antenna against high-frequency technological, thermal and chemical influences of the medium are already known from a number of patent documents. However, in no case is any measure mentioned for counteracting the unavoidably high, thermal expansion of the filling-body, in order, in such way, to prevent the consequences resulting therefrom as regards sealing, mechanical stability and the HF-performance of the horn antenna filled with dielectric material.
DE 100 40 943 A1 discloses a horn antenna for fill-level measurement, wherein the horn antenna is at least partially filled with a dielectric material.
DE 100 57 441 A1 presents a horn antenna for a radar device, whose antenna cavity is at least partially filled with a filling and/or the filling embeds the entire horn antenna. Additionally, the filling is so embodied on the process-side that it forms a flange plating as sealing element.
WO 03/078936 A1 discloses sealing between the filling-body and the horn antenna by an additional sealing element, e.g. an O-ring, and the holding of the filling-body in the horn antenna by a holding element, e.g. a coupling nut, both of which are applied in the region of the radiation aperture, or largest opening, of the horn of the horn antenna.
As already mentioned above, in none of the mentioned documents is the problem of filling material expansion or a measure counteracting such discussed. Experience has shown that pressure-stable and chemically resistant, microwave-transmissive materials do have the problem that the material strongly expands as the temperature of the surroundings rises. This unavoidable, strong thermal expansion of the filling body, caused by the large coefficients of thermal expansion of these materials, produces in the filling-body material internal mechanical stresses, which can lead to a deformation of the filling body and even a bursting of the filling-body shape. Since the filling body is bounded on almost all sides by the metal horn-antenna, and the thermally expanding, dielectric material of the filling body can only expand in the radiation direction of the horn antenna, this leads to high mechanical stresses arise in the material and especially at edges. Under continued temperature alternation, for this reason, also fatigue phenomena, such as fatigue fractures, of the material of the filling body or bordering regions of material can be observed. According to the current state of the art, the assignee manufactures the filling body from different materials in different regions. These different materials all have coefficients of thermal expansion that differ from one another. On the process-side, the filling body is most often of a thin layer of a microwave-transmissive, chemically resistant material having a high coefficient of thermal expansion, to which is joined a filling body region with a material with a low thermal expansion, e.g. Rohacell, with hollow glass spheres or with other temperature compensating fillers. The materials, whose thermal expansions are compensated by introduction of various material components and material properties, have the disadvantage that they are very expensive and, additionally, at the interfaces of the materials, a portion of the transmitted microwaves is reflected back and attenuated. Disadvantageous in the case of such multi-region filling bodies of different materials are also the complex manufacture, due to the different manufacturing processes of the separate filling body regions, and the high manufacturing costs associated therewith.